Generally, a lamp is a device for converting an electric energy into a light for objects to be recognized by workers' eyes at a dark place.
A lamp of cold cathode fluorescent tube (CCFT) is one of illumination devices for generating lights by utilizing an electric discharge phenomenon, i.e. electrons spatial movement.
These CCFT type lamps have advantages of being able to generate a white light similar to sun light, have a longer lifetime and generate less heat than fluorescent lamps and electric lamps.
This CCFT type lamp 10, as shown in FIG. 1, has a lamp tube 1 for providing a sealed discharging space, a first electrode 3 and a second electrode 5 for generating an electric discharge in the lamp tube 1.
Specifically, the lamp tube 1 has a tube body 1a, a fluorescent layer (not shown), and an operation gas 1b. More specifically, the lamp tube 1 has a closed shape sealed at both ends of the lamp tube 1. A predetermined thick fluorescent layer is formed by coating fluorescent material on inner surface of the tube body 1a, and the operation gas 1b is injected into the tube body 1a. 
On the other hand, the first electrode 3 and the second electrode 5 are formed at an inner discharging space in the lamp tube 1. The first electrode 3 and the second electrode 5 are respectively formed at one end portion and the other end portion of the tube body 1a centering about the center of the tube body 1a. An electric power is applied to a pair of first and second electrodes 3 and 5 formed in the tube body 1a. The electric power has enough power, for example, for electrons to move from the first electrode 3 to the second electrode 5.
A light generating process begins by applying an electric power to the first electrode 3 and the second electrode 5.
Accordingly, electrons spatial movements are generated from the first electrode 3 to the opposite second electrode 5. Electrons move from the first electrode 3 to the second electrode 5, and collide with the operation gas 1b. Therefore, the operation gas 1b is decomposed into atoms, neutrons, and electrons. This means that plasma is formed in the tube body 1a by electrons spatial movement.
An invisible light is generated during this process in the tube body 1a, and the invisible light stimulates the fluorescent layer (not shown). Accordingly, a white light having a wavelength of visible ray, which is recognized by eyes of workers, is generated in the fluorescent layer.
However, the lamp 10, which includes the first electrode 3 and the second electrode 5 therein, has also fatal disadvantages although the lamp has various advantages. One of the fatal disadvantages is that a luminance difference is generated between lamps 10 when a plurality of lamps 10 in parallel connected with a power supply (not shown) is driven.
On the other hand, recently, a method for forming external electrodes made of metal on the outer surface of the lamp in order to solve the problem of the luminance difference. By using the plurality of lamps manufactured by this method, the luminance difference between the lamps may be minimized when the plurality of lamps in parallel connected with a power supply is driven,
Although this method is able to solve the problem of the luminance difference, and is able to reduce the power consumption, this method causes another problem of reducing the utilization efficiency of a light because the external electrodes mask most of effective light-emitting region through which the generated light is transmitted.